Directional Overcurrent Relaying (67) Concepts
Directional Overcurrent Relaying (67) ConceptsJohn Horak, Member, IEEEBasler ElectricAbstract – Directional overcurrent relaying (67) refers torelaying that can use the phase relationship of voltage andcurrent to determine direction to a fault. There are a variety ofconcepts by which this task is done. This paper will review themainstream methods by which 67 type directional decisions aremade by protective relays. The paper focuses on how a numericdirectional relay uses the phase relationship of sequencecomponents such as positive sequence (V1 vs. I1), negativesequence (V2 vs. I2), and zero sequence (V0 vs. I0) to sense faultdirection, but other concepts such as using quadrature voltage(e.g., VAB vs IC) are included.Index Terms: directional relaying, sequence component,negative sequence, zero sequence, 67, 32, quadrature voltage.lagging current conditions rather then 1.0 power factorconditions. One approach, seen in Fig. 1, is to phase shift thevoltage signal so that the relay’s internal voltage signal(VPolarity, abbreviated as VPol) is in phase with current whencurrent lags the 1.0 power factor condition by some setting,typically between 300 and 900. The angle setting is commonlyreferred to as the maximum torque angle, MTA. In somedesigns of this concept, the current signal is skewed ratherthan the voltage signal. In some designs, other phase voltagesare used. For instance, IA could be compared to VAB, VCA,VBN, or VCN, and the detection algorithm would work, thoughthe quadrature voltage VBC gives the most independence of thevoltage signal from the effects of an A-N, A-B, or A-C fault.I. INTRODUCTIONIn some medium voltage distribution lines and almost all highvoltage transmission lines, a fault can be in two differentdirections from a relay, and it can be highly desirable for arelay to respond differently for faults in the forward or reversedirection. The IEEE device number used to signify adirectional element is either a 21 (impedance element, basedon Z V/I, and having a distance to fault capability) or a 67(directional overcurrent, generally based on the phaserelationship of V and I, with no distance to fault capability).Some applications also might use a 32 (power element, basedon P Re[VxI*]) for directional control, though in somecircumstances a 32 element may not be a good indication ofdirection to fault. This paper will review some of the mechanical, solid state, and numeric (i.e., multifunctionprogrammable logic microprocessor based) relays.II. CLASSICAL CONCEPTS FOR DIRECTIONAL ANALYSISThe classic electromechanical and solid state relay, as well assome common numeric relays, determines the direction tofault by comparing the phase angle relationship of phasecurrents to phase voltages. If only per phase watt flow (32element) is to be considered, the basic concept would be that ifIPh is in phase with VPh-N (0 , 90 ), then power flow on thatphase is indicated as forward (or reverse, depending on one’sperspective). However, for a phase to ground fault, the VPh-Nmay collapse to 0, and I may be highly lagging, so that VPh-N xIPh may be mostly VAR flow, and thus prevent the relay frommaking a correct directional decision. To resolve the lowvoltage issue, quadrature voltages (i.e., VBC vs. IA) arecommonly used. To resolve the issue that fault current istypically highly lagging, the relay current vs. voltage detectionalgorithm is skewed so that the relay is optimized to detectFig. 1. Classic VQuadrature Directional ElementThe response of the design to a phase to ground and phase tophase fault is shown in Fig. 2. The response to a phase toground fault is fairly apparent because the quadrature voltagesare assumed to be relatively unaffected by the faulted phasecurrents. However, for a phase to phase fault, the quadrature
voltages are affected. The effect is difficult to give in text. Oneshould study the diagram to develop an understanding.Basically, in the ph-ph fault, relative to ph-ground fault, notethat both Vquadrature and Ifault have shifted by 300, so there is nonet change in tendency of the element to operate.conditions as reverse fault current, as seen in Fig. 3. Anapproach to addressing this condition is to set the MTA to 300or less, so that the reverse zone reaches minimally into theforward zone.Fig. 3. Power Flow vs. MTAIII. SYMMETRICAL COMPONENTS FOR DIRECTIONALANALYSISMany modern microprocessor relays use the angularrelationships of symmetrical component currents and voltagesand the resultant angular nature of Z1, Z2, and Z0 as calculatedfrom Vphase/Iphase to determine direction to fault. These threeimpedances are used to create, respectively, three directionalassessments, 67POS, 67NEG, and 67ZERO, that are used in relaylogic in various ways by each manufacturer. There arevariations among manufacturers on of how one senses theangular relationships and, in most cases since the angularrelationship is the only concern, the magnitude is notcalculated. The common concept is that in faulted conditionsthere is an approximate 1800 difference of calculated Z1, Z2and Z0 for faults in the two directions from the relay location.This high variation in phase angle is a reliable indication ofdirection to fault.As described in detail in reference [1], the three phase voltagedrop equation for a system that can be represented by voltagesat two defined locations, (VSys and VFault in this example) isFig. 2. Phasors in Classical VQuadrature Directional ElementThe MTA setting is commonly thought of in terms of theforward-looking line impedance angle. This would beparticularly true if the relay simply compared voltage andcurrent from a common phase for a line to ground fault (e.g.,IA is compared to VAN). In this case, the relay is sensing ZAbetween the relay and the fault. However, when quadraturevoltage is used, then VPol is somewhat independent of the faultcurrent, especially for a phase to ground fault. The angle bywhich current lags quadrature voltage is a factor of bothsource impedance as well as forward-looking line impedance,so a compromise value is utilized. An MTA in the range of 300to 750 is common. When setting MTA, if an overcurrentelement is to be set below reverse direction load current, thereis a risk of the element seeing abnormal forward load VA, Sys VA, Fault Z AA VB , Sys - VB , Fault Z BA VC , Sys VC , Fault Z CA Z ABZ BBZ CBZ AC I A Z BC I B .Z CC I C (1)Again, as discussed in [1], when the impedances are highlybalanced (i.e., the diagonal self impedance elements ZAA, ZBB,and ZCC are all one value, and all off diagonal mutualimpedance elements are another value), (1) can be restated insymmetrical component quantities by the equation V0, Sys V0, Fault Z 0 V1, Sys - V1, Fault 0 V2, Sys V2, Fault 0 0Z100 I0 0 I1 .Z 2 I 2 (2)
In the typical power system, we can usually assume that, at theremote system, voltage has very low V0 and V2, and V1 is 1.0,or at least very close to 1.0. At the other end, the faultlocation, every type of fault will have differing values of V0,V1, and V2 and will need to be calculated via means that willnot be covered here (see [1]), but we know that some valueexists. Hence, (2) reduces to 0 V0, Fault Z 0 V 1, System - V1, Fault 0 0 V2, Fault 00 I0 0 I1 .Z 2 I 2 0Z10(3)If Z0, Z1, and Z2 are divided into two impedances as seen fromthe relay location (line impedance and source impedance), thenet system and associated voltage drop has the appearance ofFig. 4.Z 0,Relay Z 2, Relay V0, RelayI 0, RelayV2, RelayI 2, Relay -Z 0, Sys(6) -Z 2, Sys .(7)Note that in (6) and (7) the equations for Z0,Relay and Z2,Relay,the impedance seen by the relay will be dependent solely uponthe source impedance. (The dependency on source impedancemight be counter-intuitive to engineers accustomed to settingimpedance relays in terms of line impedances.) The angle ofZ0,Relay and Z2,Relay is the source of determining the direction toa fault. For instance, in Fig. 1, a CT polarity orientation cancause the apparent Z0 and Z2 at the relay to either match thesource impedance angle or to be inverted by 1800. The currentpolarity would be the signature of a fault that is either forwardor reverse from the relay’s location.The apparent Z1 at the relay will be dependent on the faulttype. First, we need to define the impedance between the relayand the fault location:Z1, Line , Flt Z1 from relay to line fault location(8)For a three phase faultZ1, Relay ,3 ph V1, RelayI1, Relay V1, Sys - Z1, Sys I1, RelayV1, SysZ1, Sys Z1, Line , Flt.(9) Z1, Line , FltFig. 4. Single Source System with RelayPhase to ground and phase to phase faults are morecomplicated, but have a similar derivation to (9):In this application, (3) can be restated as 0 V0, Fault V1, Sys - V1, Fault 0 V2, Fault Z 0, Sys 0 0 0Z1, Sys0Z1, Relay , Ph - Ph (4)0 Z 0, Line 0 0Z 2, Sys 00Z1, Line00 I 0, Relay 0 I1, Relay Z 2, Line I 2, Relay The voltage division of (4) allows us to calculate the voltage atthe relay by starting at the fault location and working back tothe system or starting at the system and working toward thefault. Since we do not know the fault voltages, we need to takethe latter approach, so we can calculate relay voltage from V0, Relay 0 Z 0, Sys V1,Relay V1, Sys - 0 V2, Relay 0 0 0Z1, Sys00 I 0, Relay 0 I1, Relay .Z 2, Sys I 2,Relay (5)If we solve (5) for the impedances, since V2,Sys 0 and V0,Sys 0, thenZ 0, Sys Z 0, Line, Flt Z1, Line , F Z 2, Sys Z 2, Line , FltZ1, Relay , Ph - N Z 2, Sys Z1, Line , Flt Z 2, Line, Flt .(10)(11)In these cases, the Z1 measurement as seen at the relay is amix of the various system and line impedances, but it tends tobe a measure of forward looking line impedance more thansource impedance, most clearly seen in (9). The impedanceangle of the various components of Z1,Relay tend to be similar,in the area of 500 - 850. The calculated angle of Z1,Relay againhas a 1800 phase angle reversal depending on direction to thefault, which is the signature of a fault that is either forward orreverse.If a relay uses Z1,Relay for sensing direction to fault, the Z1,Relaymeasurement will see balanced load flow as an indication ofthe direction to fault and, hence, to turn on overcurrentelements (67/51) that are set to look in the direction of presentload flow. The Z1 that is sensed during balanced load flowconditions is a minor modification of (9):Z1, Relay Z1, Line Z1, Load .(12)
The angle of Z1 can be a poor indicator of fault location. Forinstance, when a customer’s DG (distributed generator) existsto peak shave, it has the ability to control the power factor atthe PCC (point of common coupling). Power swings thatoccur in post fault conditions give transient VA flow at almostany angle. On a more steady state basis, a DG that runs tokeep power at the PCC near zero could cause net power factor,and hence the angle of Z1, to be almost any value.would fall into either the forward or reverse zone, dependingon relay setup and CT connections. Note that in an impedanceplot, MTA is counterclockwise from the reference R axis, ascompared to the classical approach of showing I relative to Vwhere MTA is clockwise from the reference VA axis.Directional overcurrent relaying would not be useful in asystem with only one source. A system with two sources isshown in Fig. 5. We can apply the same concepts as above toanalyze the circuit. We have a fault location where there is acalculable level of sequence voltages, and system and lineimpedances in two directions, looking back toward the systemsource voltage. The same approach as in (1) to (7) can beapplied to find the impedance seen by the relays at either endof the line.Fig. 5. Two Source SystemThe impedance as seen by relay A will vary according to thedirection to the fault. For faults on the two different sides ofthe breaker, the relay will sense two completely differentimpedances. For fault FA,F and FA,R the impedance seen byrelay RA will be:Z 0, Relay , Fault A, For -Z 0, Sys , AZ 0, Relay , Fault A, Rev 1 1800 ( -Z 0, Sys , B - Z 0, Line ) ( Z 0, Sys , B Z 0, Line ).Z 2, Relay , Fault A, For -Z 2, Sys , A(13)Z 2, Relay , Fault A, Rev 1 1800 ( -Z 2, Sys , B - Z 2, Line ) ( Z 2, Sys , B Z 2, Line )In (13), Z#,Line refers to the entire line impedance. The 1 1800factor in (13) accounts for the effective change in CT polarityfor faults in the reverse direction. The positive sequenceimpedance does not lend itself to simple equations such as(13), but for 3 phase faults and unfaulted load flow conditionsZ1,Re lay , Forward , Faulted Z1, Line , FltZ1,Re lay ,Re veverse , Faulted Z1, Sys , A,to FaultZ1,Re lay , Forward ,Unfaulted Z1, Line Z1, Load , B.(14)Z1,Re lay ,Re verse ,Unfaulted Z1, Sys , A Z1, Load , AA graphical representation of the forward and reverse zones ofprotection can be seen in Fig. 6. The MTA is a user settingthat effectively defines forward and reverse phase angles.Sensed impedance angles that are /- 900 from the MTAFig. 6. Forward and Reverse Impedance AnglesAs previously mentioned, the various relay manufacturershave differing ways of sensing angular relationships. The mostobvious process is to measure the impedance angle andcompare it to a window of the MTA /-900 as forward orreverse. There are alternate processes in use. For instance, asseen in Fig. 7, one manufacturer configures its Z2,Relay andZ0,Relay directional elements to subtract the MTA from thecalculated impedance, and then find the real portion of thisresultant impedance, ZReal, which can be a positive or negativevalue. Then, ZReal is compared to user settings for the decisionpoints for forward and reverse. For the great majority of casesthis gives the same result as the phase angle window.
and 67/51Q (negative sequence) elements and similar 67/50elements, and that each has a forward or reverse looking modewith different settings for each direction. There are threedirectional elements called the 67POS (positive sequence),67NEG (negative sequence), and 67ZERO (zero sequence) thatcontrol the 67/51 and 67/50 elements. The protective elementsand their directional controls are:A given relay may have more than one copy, or no copy, ofthe indicated element, and a given relay may or may not givethe user direct access to 67POS, 67NEG, and 67ZERO.Fig. 7. Alternate Process to Using Phase AngleTABLE 2 - TYPICAL DIRECTIONAL ELEMENTSIV. VARIATIONS OF ZERO SEQUENCE DIRECTIONALITYZero sequence directionality has several variations. The V0and I0 used in a Z0 measurement each can be obtained fromvarious inputs: V0 as calculated from the 3 phase VT inputs V0 as seen on a 4th auxiliary VT input on the relay (VXbelow). This VX input can be connected to a variety ofsources, such as:o a broken delta VT, oro the neutral of an impedance grounded generator. I0 as calculated from the 3 phase CT inputs. I0 as seen on a 4th CT input on the relay (IG below). Thisauxiliary CT input can be connected to a variety ofsources, such as:o a window CT that wraps all 3 phases,o a window CT that wraps all 3 phases as well as apower carrying neutral conductor,o the neutral of a transformer, oro the neutral of a generator.The result is that there are 5 different combinations of currentsand voltages that can be used to create a directional 67ZEROThe last item in Table 1 uses only current for the directionaldecision and is sometimes referred to as zero sequence currentpolarization. The MTA is always 00. If the two currents are inphase ( /-900), the fault is forward.TABLE 1 - VARIATIONS OF ZERO SEQUENCE POLARIZATION67ZEROType Quantity 1Quantity 2V0I0Calculated V0 from phase VTsCalculated I0 from phase CTsV0IGCalculated V0 from phase VTsCurrent on 4th CT inputVXI0Voltage on 4th VT inputCalculated I0 from phase CTsVXIGVoltage on 4th VT inputCurrent on 4th CT inputI0IGCalculated I0 from phase CTsCurrent on 4th CT inputV. OVERCURRENT AND DIRECTIONAL ELEMENT NAMES ANDCONTROLOne needs to understand which directional decision controlswhich overcurrent element. There is no standard way to nameall of the overcurrent elements that are involved. Assume forthe discussion that there are 67/51P (phase), 67/51G (ground),67/51 Elements67/50 ElementsDirectionally Controlled by(typically)67/51P - Forward67/50P - Forward67POS or 67NEG Forward67/51P - Reverse67/50P - Reverse67POS or 67NEG Reverse67/51G - Forward67/50G - Forward67ZERO or 67NEG Forward67/51G - Reverse67/50G - Reverse67ZERO or 67NEG Reverse67/51Q - Forward67/50Q - Forward67NEG Forward67/51Q - Reverse67/50Q - Reverse67NEG ReverseVI. OTHER ISSUESThere is a number of subtleties involved in the forward/reversedirection decision and element operation that will not becovered here. One should refer to the various relaymanufacturers’ instruction manuals for details on their relays’algorithms. Some issues that need to be understood that canvary by manufacturer implementation:A. Memory PolarizationFor close-in three phase faults, the voltage at the relay mayfall to near 0. Due to the low voltage, the relay’s 67 logiccannot be relied upon to make a correct directional analysisdecision, and in some relay configurations, if the relay cannotdetermine forward or reverse, it does not trip at all. To addressthis issue, numeric relay manufacturers create a memorypolarization scheme. The relay constantly is reading thepresent voltage and using it to create a voltage vector (V1). If afault occurs that suddenly drives voltage too low to be used fordirectional analysis, the relay reaches back to its memory andprojects the past voltage vector into the present. The V1voltage vector change very slowly in the normal powersystem, so a past V1 voltage vector is a good indication of thevoltage vector that would exist if a fault had not occurred, andit is a reliable backup for directional analysis.B. Close in to Fault LogicWhen a breaker is closed into a three phase fault (i.e.,grounding chains), the memory polarization scheme will notwork because there is no pre-event V1 vector for the relay towork with. The Close In To Fault logic monitors for a breakerclose and enables a high set three phase non-directionalovercurrent sensing circuit for a short period of time. Thesetting of the 50 element must be above maximum load inrushin either direction.
C. Superimposed ComponentsWhen heavy load flow occurs at the same time as a low levelfault, it can confuse a directional element. This situation willbe seen in the application discussed in section VIII. Somemanufacturers have implemented a scheme that tries toseparate out load flow currents from fault currents, usingschemes referred to as superimposed components. It is similarin application to memory polarization but involves bothcurrent and voltage from the past into the present, rather thanjust the voltage. Assume steady state load flow conditions.Assume a sudden change, due to a fault, is seen. The relay cantake the voltage and current from the past several cycles,before the fault, and project it into the present. This projectedvoltage and current is compared to actual faulted voltage andcurrent. This scheme allows the fault current to be separatedfrom the overriding load current and, hence, improve thedecision about where the fault is located. The algorithms needto include intelligence and logic to differentiate normalswitching events from fault events.D. Minimum SensitivityA relay has limits to its sensitivity. There must be sufficientquantities of current and voltage for a directional decision.The minimum quantity varies by manufacturer. The responseof the relay to low voltage or current varies, but typically, therelay will default to a “neither forward no
Abstract – Directional overcurrent relaying (67) refers to relaying that can use the phase relationship of voltage and current to determine direction to a fault. There are a variety of . (9) Phase to ground and phase to phase faults are more complicated, but have a similar derivation to (9): 1, , -
Three-phase Directional Overcurrent (67) Each one of the three-phase overcurrent stages of the P127 can be independently configured as directional protection with specific relay characteristic angle (RCA) settings and boundaries. Each directional stage has instantaneous (start) forward/reverse outputs available. Directional Overcurrent Tripping .
vide directional ground fault protection in the KD-4 carrier relaying scheme. Operation of the relays in connection with the carrier scheme is fully described in I.L. 41-911. CONSTRUCTION The type KRD-4 directional overcurrent ground relay consists of a dual polarized directional unit, an instantaneous overcurrent unit, and an indicating
diversity protocols for single-relay scenarios and analyzed their outage performance. Specifically, the authors in [3] proposed fixed and adaptive relaying protocols. Examples of fixed relaying are amplify-and-forward and decode-and-forward relaying. Adaptive relaying protocols comprise selection relaying, in which the relay applies threshold tests
Distribution System Feeder Overcurrent Protection figuration. Overcurrent relaying in one form or another has been used for relaying of all system components. It is now used primarily on distribution systems where low cost is an important factor. Figure 3 shows a family of inverse-time curves of the widely used IAC relay, which is an induction .
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Opportunistic Resource Allocation and Relaying Methods for Quality of Service in the Downlink of . "Relaying in Long Term Evolution: Indoor full frequency reuse," Proceedings of IEEE European Wireless Conference 2009, pp. 298-302, Aalborg, 17-20 May 2009. 2. V. Venkatkumar, T. Haustein and M. Faulkner, "Relaying results for indoor .
21 Phase Distance 51V Voltage Controlled/Restrained Overcurrent 24 Volts per Hertz 32 Reverse Power 40 Loss of Field 46 Negative Sequence Overcurrent 50/51 Instantaneous/Time Delayed Overcurrent 50/51GN Instantaneous/Time Delayed Overcurrent in Gen Neutral 51TN Time Overcurrent Relay in GSU Neutral 59 Overvoltage 59GN Ground Overvoltage
ASTM E 989-06 (2012), Classification for Determination of Impact Insulation Class (IIC) ASTM E 2235-04 (2012) Standard Test Method for Determination of Decay Rates for Use in Sound Insulation Test Methods. Test Procedure. All testing was conducted in the VT test chambers at Intertek-ATI located in York, Pennsylvania. The microphones were calibrated before conducting the tests. The airborne .
